1. Field of the Invention
This invention relates to ink jet recording and particularly to a method and apparatus for generating a stream of drops for use in an ink jet printer.
2. Description of Prior Art
In ink jet recording it is well-known to produce a stream of liquid ink under pressure and to produce perturbations in the stream to cause it to break up into individual uniformly spaced drops which are then directed in a controlled manner onto a record medium to visually record the information. The perturbations can be formed by electromechanical devices which vibrate the jet-forming elements or by the application of external fields to the unsupported jet stream which produce perturbations in the jet stream. U.S. Pat. No. 3,596,275, issued July 27, 1971 to Richard G. Sweet, shows using either a magnetostrictive vibrator or an excitational electrode for producing drops from a conductive ink jet. In U.S. Pat. No. 3,298,030, issued on July 12, 1965 to Arthur M. Lewis and Arling D. Brown, Jr., a piezoelectric transducer is used as the perturbation-producing means. In the previously-mentioned application, Ser. No. 429,414 of George J. Fan and Richard A. Toupin, drops are formed in a magnetic ink jet stream using externally-applied magnetic fields at plural uniformly-spaced positions along the stream, the spacing of the field-producing elements being equal to the wavelength of the perturbations produced in the stream or a multiple thereof.
One of the problems associated with previous drop generators has been the fact that as the stream breaks up into individual drops there is a tendency for satellites to form. The precise explanation of why satellites form is not fully understood; however, it has been observed that satellite drops, when formed, will usually form from the ligament portions of the jet stream which connect the varicosities produced by the perturbations. It has also been observed that the satellites can have a velocity equal to or different from the adjacent large drops. Depending on the relative velocity of the satellite and large drops merging will take place if their relative velocities are different. The rate at which merging takes place, however, can affect the control of the droplets and the print quality or contamination of the ink jet apparatus.
U.S. Pat. No. 3,683,396, issued Aug. 8, 1972 to Robert I. Keur, Sandra Miller and Henry A Dahl, attempts to solve the satellite problem by designing the nozzle to have fluid resonance to obtain the formation of fast satellites. The nozzle is designed so that its internal length is determined in relation to the speed of sound to the fluid in the nozzle and the desired frequency of resonance.
U.S. Pat. No. 3,334,351, issued Aug. 1, 1967 to Norman L. Stauffer, shows a method of merging satellite drops by disturbing the stream to impart a rolling motion to ink drops through the use of dual vibration means operated transverse to and in the direction of flow of the jet stream.
The previously-mentioned application of Joseph P. Pawletko and Bruce A. Wolfe shows a mechanical structure in which two piezoelectric devices operate in different modes on a cantilever beam to prevent formation of satellite drops by imparting a spin thereto.
It will be appreciated that the prior art solutions for eliminating or merging satellite drops require specialized complex structures. Furthermore, such structures lack versatility, since the mechanical devices once designed are strictly confined to specific operating conditions having a very narrow range. As the conditions of the ink and the operating properties of the system vary, the effectiveness of prevention or merging of satellites degrades considerably and the means for controlling the variation in operating conditions becomes complex and costly.